It’s said that Imam Ali (as) was killed because of his excessive emphasis on practicing of and adhering to justice. After the demise of the third Caliph, people thronged Imam Ali (as) to accept the reins of Islamic government.
In sermon 3 of Nahjul Balagha:
“At that moment, nothing took me by surprise, but the crowd of people rushing to me. It advanced towards me from every side like the mane of the hyena so much so that Hasan and Husayn were getting crushed and both the ends of my shoulder garment were torn. They collected around me like a herd of sheep and goats,” he said.
There were overwhelming crowds who were pleading to him to accept the caliphate and become their ruler. When the people were taking oaths of allegiance to Imam Ali (as), their chant was “None except Ali is fit for the caliphate.” The atmosphere was filled with joy and happiness. Every one was embracing the name of Imam Ali (as) with such a heavy satisfaction.
But this overwhelming support was short lived. Why?
Because as Imam Ali (as) took reins and started ruling according to the rules dictated by the holy Qur’an and Sunnah of the holy Prophet (s). Many people who took the oath of allegiance sensed a lingering danger to their unjustly amassed wealth. Though the people did want a good and virtuous ruler, they did not want an absolute just one who will not show any leniency or favoritism in the matters of administering justice and fairness.
When Imam Ali (as) started administering justice and running the affairs of Islamic government as per the divine law, many powerful people started deserting him. In the same sermon quoted above Imam (as)mentions the breaking of oaths of people:
“When I took up the reins of government one party broke away and another turned disobedient while the rest began acting wrongfully as if they had not heard the word of Allah saying:
‘That abode in the hereafter, We assign it for those who intend not to exult themselves in the earth, nor (to make) mischief (therein); and the end is (best) for the pious ones.’ (Qur’an, 28:83)”
Imam Ali (as) strictly ordered his officers, administrators, and provincial governors to abide by justice. He punished and dismissed the corrupt administrators and governors. In a letter to Ziyad b. Abih, Imam Ali (as) reprimands Ziyad against corruption:
“I truthfully swear by Allah that if I come to know that you have misappropriated the funds of the Muslims, small or big, I shall inflict upon you such punishment which will leave you empty handed, heavy backed and humiliated…” (Nahjul Balagha, Letter 20).
He advises his governors to establish justice and eradicate corruption. In advice to Malik Ashter when the latter was appointed the governor of Egypt, the Imam (as) writes:
“Do justice for Allah and do justice towards the people, as against yourself, your near ones and those of your subjects for whom you have a liking because if you do not do so you will be oppressive, and when a person oppresses the creatures of Allah then, instead of His creatures, Allah becomes his opponent, and when Allah is the opponent of a person He tramples his plea; and he will remain in the position of being at war with Allah until he gives it up and repents. ”(Nahjul Balagha, Letter 53).
He strictly forbade his governors and officers against accepting bribes and gifts:
“Any ruler who hides himself from people’s needs, Allah will also hide Himself from him and his needs on the Judgment day; and if he accepts gifts, he would be a traitor; and if he accept bribes, he would be a polytheist.” 
In Al-Kafi, related by Abu Mikhnaf:
“A group of the Shi’ites came to Imam Ali (as) and said: ‘O Amir al-Momineen! Would that you distributed these assets among the dignitaries and chiefs and give them priority to us so that things settle down. Then you would return to the best of features that Allah has accustomed you to; that is, making equal distributions, and practicing justice among citizens.’
Amir al-Momineen (as) said, ‘Woe on you! You are commanding me to seek support by doing injustice and inequity to the Muslims over whom I have been chosen as a guardian. By Allah, I will not do that as long as the world keeps going on and as long as I see stars in the stars. By Allah, even if it were my property, I would distribute it equally among them; how would it be when the property is theirs?'”
Many people who previously were with Imam Ali (as) deserted him and joined ranks with his enemies only because they could not withstand his justice. When Imam Ali (as) punished them for corruption or wrong doings, or when they felt that the unwavering rule of justice is going to snatch from them of what they have amassed unjustly and illegally, they rose against the Imam and joined the ranks in Damascus with Muawiya bin Abu Suffiyan.
We have to think that when our Imam (as) did not show any leniency towards his own blood relatives or friends in terms of administering justice, then, what good we have in ourselves that we’ll be let go away with injustice in our lives and in our acts?
The Ali of this age is alive and is waiting to receive order from His Lord to reappear. He (aj) will also do what his grandfather Imam Ali (as) did: no compromise in administering justice.
Before we shout loud slogans of Ali..Ali.., we should think seriously and ponder on the fact that how much justice we can accept against our own selves. People before us left Ali of their age because they could not bear justice against their selves. Are we really ready to welcome Ali of our age?
1. Thawab al-A’mal: 310/1. Bihar al-Anwar: 72/345/42 , as quoted in Imam Ali’s (as) Book of Government by Ayatullah Muhammadi Rayshahri